1. Field of the Invention
The present invention relates to a light-sensitive device for separately detecting light of two different wavelength bands.
2. Description of the Prior Art
In optical data communication systems, there are times when two independent data streams are to be transmitted over one data channel (e.g., optical fiber) in different wavelength bands and then are to be processed separately.
In such systems, the two data streams are commonly separated by means of optical devices, such as prisms, color filters or interference filters. A wavelength-demultiplexing light-sensitive device is described on pages 388 and 389 of the June 21, 1979 issue of the journal "Electronics Letters". There, a number of layers are deposited on a substrate using mesa technology to form two pn junctions which are optically in series, and a filter interposed between these pn junctions. The first pn junction responds only to photons of higher energy, i.e., to light of shorter wavelength, than the second. The intermediate filter is a semiconductor layer whose absorption edge lies between the two wavelengths to be detected separately. An arrangement of this kind can detect light at different wavelengths without additional means, but it is difficult to build and not sufficiently selective.